Process and device for writing by ink jet

ABSTRACT

A jet of ink, formed of a succession of discrete droplets, is extracted from a nozzle by an electric field and is passed between deflecting electrodes towards a writing surface. A control voltage applied across the deflecting electrodes deflects the jet to trace signs on the surface, these signs having discontinuities or gaps corresponding to abrupt changes in the control voltage. A measurement of variations of the voltage of the nozzle is used to supply pulses corresponding to the departure of individual droplets from the nozzle. These pulses are delayed so that each delayed pulse corresponds to an instant when no droplet is located between the deflecting electrodes, and the delayed pulses trigger the application of abrupt changes in the control voltage.

United States Patent Ascoli et al. 1 July 1, 1975 PROCESS AND DEVICE FOR WRITING BY 3.725.900 4/1973 Ohmann 346/75 X INK JET [75] Inventors: Enzo Ascoli, Lausanne; Raymond Primary Examiner-1956p! Hartary Pidoux, puny; Marc Perdrix, Attorney, Agent, or F1rmEm0ry L. Groff. Jr.

Champagne, all of Switzerland [73] Assignee: Paillard S.A., Sainte croix (Vaud), [57] ABSTRACT Switzerland A jet of ink, formed of a succession of discrete drop- [221 Filed: Aug 14 1973 lets, is extracted from a nozzle by an electric field and is passed between deflecting electrodes towards a writ- [211 App]. No.1 38 ing surface. A control voltage applied across the deflecting electrodes deflects the jet to trace signs on the [30] Foreign Application Priority Data surface, these signs having discontinuities or gaps corresponding to abrupt changes in the control voltage. A l2, swltzerland measurement of variations of the voltage of the nozzle is used to supply pulses corresponding to the depar- [fi] 343/011,?irgl'll(55 ture of individual droplets from the nozzle. These [58] Fnid 346 75/ l pulses are delayed so that each delayed pulse c0rre 1 le 0 Earl: sponds to an instant when no droplet is located between the deflecting electrodes, and the delayed [56] References C'ted pulses trigger the application of abrupt changes in the control voltage 3.060,429 10/1962 Winston 346/75 x 3,466,659 9/1969 Ascoli 346/140 8 Clam! 1 Drawmg Flgure PROCESS AND DEVICE FOR WRITING BY INK JET The invention relates to writing by means of a jet of ink composed of a succession of discrete droplets of ink.

In known writing processes and devices, droplets of ink are extracted from a delivery nozzle by the application of an electric field, these droplets forming a jet of ink directed towards .a writing surface. The jet of ink passes between deflecting electrodes submitted to electrical voltages for deflecting the droplets to trace signs, these applied voltages having abrupt variations at discontinuities in the signs to be traced, i.e. at discontinuities in individual signs, during passage from one sign to another, and when the jet of ink is transferred from a target to the writing surface, or vice versa, at the beginning or the end of a period of writing after or before by a stopping time. An example of such process and apparatus is shown in the U.S. Patent to Winston, U.S. Pat. No. 3,060,429, issued Oct. 23, 1962.

In these processes and devices, the delivery of the droplets of ink can be carried out at a given frequency, for example under the control of a vibrator acting on the delivery nozzle. In other processes and devices, the frequency of delivery of the droplets is simply allowed to be set up freely. Upon an abrupt change of the voltage applied to the deflecting electrodes, for example when the trace ofa sign to be written includes a discontinuity, or corresponding to a gap between one sign and the following one, or at the beginning or at the end of a period of writing, a droplet of ink may be in transit between the deflecting electrodes. In this eventuality, the first part of the trajectory of the droplet between the deflecting electrodes corresponds to a given point of a sign to be traced, whereas the second part of its trajectory between the electrodes corresponds to a different point. The result is that the droplet adopts an intermediary final trajectory and consequently impinges against the writing support, or sheet on which the signs are traced, at a point which should normally have remained blank. Consequently, the writing sheet is not immaculate and has numerous speckles which, despite being very small, nevertheless prevent an excellent presentation from being obtained.

An aim of the present invention is to avoid the production of such stray droplets and the small speckles which result.

According to the invention, a process for writing by means of a jet of ink comprises: applying an electric field to extract a succession of discrete droplets of ink from a delivery nozzle to form ajet of ink directed generally towards a writing surface and passing between deflecting electrodes; applying a control voltage across the deflecting electrodes to deflect the jet of ink to trace signs on the writing surface, said control voltage having abrupt variations corresponding to discontinuities in the signs being traced determining the individual passages of the droplets and making said abrupt variations in the control voltage coincide with instants when no droplet is located between said deflecting electrodes.

The invention also concerns a device for writing by means of a jet of ink, comprising an ink-delivery nozzle,

a writing surface, deflecting electrodes between the nozzle and the writing surface, means for applying an electric field to extract a succession of discrete droplets of ink from the nozzle to form a jet of ink directed generally towards the writing surface and between the deflecting electrodes, means for applying a control voltage across the deflecting electrodes to deflect the jet of ink to trace signs on the writing surface, said control voltage having abrupt variations corresponding to discontinuities in the signs being traced, means for determining the individual passages of the droplets, and means for making said abrupt variations in the control voltage coincide with instants when no droplet is located between said deflecting electrodes.

Preferably, said means for determining the individual passages of the droplets include means for measuring variations of potential of the nozzle.

The invention will now be particularly described, by way of example, with reference to the single FIGURE of the accompanying drawings which is a schematic diagram of an embodiment of a device according to the invention.

Referring to the drawing, a delivery nozzle 1, to which electrically conductive ink is supplied by a conduit 2 connected to an ink reservoir, not shown, delivers a jet of ink in the form of a succession of discrete droplets 3 which are accelerated and extracted from the nozzle 1 by an electric field produced between nozzle 1 and an annular electrode 4. The jet of droplets 3 is directed generally towards a writing sheet placed on a cylindrical support 5. During their passage from the nozzle 1 towards the support 5, the droplets 3 pass between deflecting electrodes 6a and 6b, across which a control voltage is applied to deflect the droplets so that they either impinge on the writing sheet or on a recuperation target 15. The length of the facing electrodes 6a and 6b along the direction of the jet of droplets 3 is chosen so as to be less than the distance separating successive droplets 3 of the jet in the region of the electrodes. The electric field accelerating the droplets 3 is produced by a D.C. generator 7 which is connected to the nozzle 1 by a resistor R The generator 7 supplies a voltage of the order of 2,500 volts, to create an electric field to accelerate the droplets 3 toward the writing sheet on cylindrical support 5. Reference may be had to the Winston patent, referred to above, for a more detailed explanation of this effect. The approximate diameter of the droplets is 7 X 10. The spacing or distance between two successive droplets is on the order of 3 to 4 mm. The field of deflection may reach 1,500 V/cm. The length of deflecting plates (electrodes) 6a, 6b, is 1.2 mm.

The specific numerical parameters or constants would differ with different writing heads having different characteristics. In the present example, the voltage of 2,500 volts between the nozzle 1 and electrode 4 will produce a speed approaching 7 meters per second for the droplets. The nozzle characteristics and the properties of the ink, especially its electrical conductivity, its surface voltage and its velocity, may be chosen to produce a droplet frequency of approximately 2,000 drops per second. The distance between nozzle 1 and electrodes 6a, 6b is about 3.5 mm, which, for the speed of 7 meters per second, gives a transition time of about 0.5 milli-second for the passage of one droplet from the nozzle to the entry of electrodes 6a, 6b. The distance between two successive droplets can be determined from the speed and the frequency, and in this example is approximately 3.5 mm.

Obviously a nozzle must be used in which the droplets have a substantially constant point of departure. A nozzle of the type shown in the US. Pat. No. 3,466,659, issued Sept. 9, 1969, to E. Ascoli, a coinventor in this application, is used, wherein the droplets are formed immediately at the nozzle opening.

In fact, if one wishes to be completely accurate, the ink droplets do not detach from the nozzle tip in an extremely precise manner. Measurements show that the frequency which we have already indicated as being equal to 2,000 cycles per second varies by about :2 percent. This variation of frequency corresponds, in fact, to a variation in the place where the separation of the droplet occurs. For the already-indicated speed of 7 meters per second, a distance of 3.5 mm between droplets is obtained, and this 2 percent variation in this distance gives a deviation of i007 mm in relation to the ideally accurate point of detachment of the droplets.

lt is quite obvious that, considering the other dimensions of the apparatus, especially the efficient width of the deflecting plates, which is 1.8 mm, the extremely slight inaccuracy of 0.07 mm of the droplet formation point is entirely negligible. As a matter of fact, the purpose of the invention is to prevent a droplet of ink, which is spaced from the preceding and following droplets by 3.5 mm, from being found, at the moment of the abrupt change of voltage, in the electrostatic field of deviation, the width of which is only 1.8 mm. The accuracy needed to attain this result is equal to the difference between the distance separating the droplets and the width of the electrostatic field, or

3.5 mm 1.8 mm= 1.7 mm

It can thus be seen that this slight inaccuracy of 0.7 mm is insignificant, since the required accuracy is only 10.85 mm.

To determine the individual passage of the droplets 3 between the electrodes 6a and 6b, the departure of each droplet from the nozzle 1 is detected. The extraction of each droplet corresponds to the removal of an electric charge from the nozzle 1 and therefore to a very slight variation in its potential. In fact, this variation is of the order of one teh-thousandth of the DC voltage supplied by generator 7. The variation in potential of nozzle 1 at the departure of each droplet originates basically from the electrical capacity of the nozzle in relation to the ground. This capacity is on the order of several pF. The departure of each droplet corresponds to the departure of an electrical charge (about 2 X 10 coulomb). Since the voltage ofa condenser is equal to the product of its capacity multiplied by the charge, it is clear that a decrease in the charge causes a corresponding decrease in voltage. It is quite obvious that this decrease in charge is compensated by a current passing in resistor R However, this current progressively recharges nozzle 1, while the departure of a droplet is an extremely sudden and quasiinstantaneous phenomenon. The variation in potential on the nozzle, then, has the aspect of a sawtooth curve. The phenomenon of the formation of droplets of regular dimensions-has been studied and known since the end of the last century.

To detect these variations of potential, the voltage between the nozzle 1 and earth is taken by a capacitor C, and a resistor R and fed to an input ofa differential amplifier 8. Because of the very small value of the said voltage variations, it is necessary to eliminate any stray voltage which could appear on the nozzle 1 and, for this purpose, a second input of amplifier 8 is fed from a line including a resistor R a capacitor C and a resistor R Resistors R and R have the same value, and the same is true for resistors R and R,. In this manner all of the variations of voltage appearing at the terminals of generator 7 are transmitted with the same amplitude to the two inputs of the amplifier 8 and thus give no differential output signal. However, the variations of voltage of the nozzle 1 itself supply a signal to only a single input of the amplifier 8 and thus produce a differential output signal.

Pulses, which correspond to the departure of each droplet 3 from the nozzle 1, are thus obtained at the output of the amplifier 8. These pulses are shaped by a Schmitt trigger 9 and supplied to a delay circuit 10 which provides a delay substantially equal to the time between the instant when one droplet leaves the nozzle 1 and an instant when two successive droplets are in positions 3' and 3" symmetrically disposed about the electrodes 6a and 6b, i.e. when no droplet is located between these electrodes.

The output signal of the delay circuit 10 is supplied to a circuit 11 which is monostable trigger setting a given duration for each signal, this duration being substantially less than the time of transit of a droplet from one, 3', of the said symmetric positions to the other, 3". The output signal of circuit 11 is applied, after inversion, to a terminal CP of a delay store or delay flipflop 12, whose input D receives a deflection order from a circuit 13, and whose output Q must transmit this deflection order via an amplifier 14 to the electrodes 6a and 6b. The delay circuit 10 is an electronic device which may be calculated by any person skilled in the art. Such delay circuits are known separately, so that their construction has no direct relationship with the present invention.

In this manner, when the circuit 13 delivers a deflection order corresponding to an abrupt jump of the voltage between the electrodes 6a and 6b, this order is recorded in the store 12 and is only transmitted to the deflection electrodes after the arrival of a pulse at CP, which indicates that there is no droplet in transit between the electrodes 6a and 6b.

Of course, various modifications of the described device may be provided, notably detection of the presence or absence of droplets 3 between the electrodes 6a, 612 by different means. It is, for example, possible to employ the interception of a luminous beam by the droplets, this beam being directed towards a photoconductor. However, such a solution is delicate because of the extreme minuteness of the droplets.

In the case where delivery of the droplets is triggered by a vibrator, the freguency of supply of the droplets is well defined so that it is possible, by observation of the phase of the vibrator, to determine at which moment there is no droplet between the electrodes 6a and 6b.

We claim:

1. A process for writing by means of ajet of ink, comprising: applying an electric field to extract a succession of discrete droplets of ink from a delivery nozzle to form a jet of ink directed generally towards a writing surface and passing between deflecting electrodes; applying a control voltage across the deflecting electrodes to deflect the jet of ink to trace signs on the writing surface, said control voltage having abrupt variations corresponding to discontinuities in the signs being traced, determining the individual passages of the droplets and making said abrupt variations in the control voltage coincide with instants when no droplet is located between said deflecting electrodes.

2. A process according to claim 1, in which the departure of each droplet from the delivery nozzle is detected by measuring variations of potential of the nozzle.

3. A device for writing by means of a jet of ink, comprising an ink-delivery nozzle, a writing surface, deflecting electrodes between the nozzle and the writing surface, means for applying an electric field to extract a succession of discrete droplets of ink from the nozzle to form a jet of ink directed generally towards the writing surface and between the deflecting electrodes, means for applying a control voltage across the deflecting electrodes to deflect the jet of ink to trace signs on the writing surface, said control voltage having abrupt variations corresponding to discontinuities in the signs being traced, means for determining the individual passages of the droplets, and means for making said abrupt variations in the control voltage coincide with instants when no droplet is located between said deflecting electrodes.

4. A device according to claim 3, in which said means for determining the individual passages of the droplets include means for measuring variations of potential of the nozzle.

5. A device according to claim 4, in which said measuring means comprises a differential amplifier having two inputs supplied by signals coming respectively from the delivery nozzle and from a voltage generator which applies a potential to the nozzle to extract and accelerate the droplets, said amplifier having an output supplying signals representing variations of the potential of the nozzle.

6. A device according to claim 5, comprising a delay circuit supplied by the said amplifier, the delay of said circuit being substantially equal to the time between the instant when a droplet leaves the nozzle and an instant when no droplet is located between the deflecting electrodes.

7. A device according to claim 6, comprising a monostable trigger piloted by the delay circuit and supplying a pulse the duration of which is substantially less than the time of transit of a droplet between the deflecting electrodes.

8. A device according to claim 7, comprising a delay store receiving orders for abrupt variations of the deflecting voltages, the store being freed" by pulses from the monostable trigger to enable the application of the abrupt voltage variations to the electrodes during periods when no droplet is in transit between the deflecting electrodes. 

1. A process for writing by means of a jet of ink, comprising: applying an electric field to extract a succession of discrete droplets of ink from a delivery nozzle to form a jet of ink directed generally towards a writing surface and passing between deflecting electrodes; applying a control voltage across the deflecting electrodes to deflect the jet of ink to trace signs on the writing surface, said control voltage having abrupt variations corresponding to discontinuities in the signs being traced, determining the individual passages of the droplets and making said abrupt variations in the control voltage coincide with instants when no droplet is located between said deflecting electrodes.
 2. A process according to claim 1, in which the departure of each droplet from the delivery nozzle is detected by measuring variations of Potential of the nozzle.
 3. A device for writing by means of a jet of ink, comprising an ink-delivery nozzle, a writing surface, deflecting electrodes between the nozzle and the writing surface, means for applying an electric field to extract a succession of discrete droplets of ink from the nozzle to form a jet of ink directed generally towards the writing surface and between the deflecting electrodes, means for applying a control voltage across the deflecting electrodes to deflect the jet of ink to trace signs on the writing surface, said control voltage having abrupt variations corresponding to discontinuities in the signs being traced, means for determining the individual passages of the droplets, and means for making said abrupt variations in the control voltage coincide with instants when no droplet is located between said deflecting electrodes.
 4. A device according to claim 3, in which said means for determining the individual passages of the droplets include means for measuring variations of potential of the nozzle.
 5. A device according to claim 4, in which said measuring means comprises a differential amplifier having two inputs supplied by signals coming respectively from the delivery nozzle and from a voltage generator which applies a potential to the nozzle to extract and accelerate the droplets, said amplifier having an output supplying signals representing variations of the potential of the nozzle.
 6. A device according to claim 5, comprising a delay circuit supplied by the said amplifier, the delay of said circuit being substantially equal to the time between the instant when a droplet leaves the nozzle and an instant when no droplet is located between the deflecting electrodes.
 7. A device according to claim 6, comprising a monostable trigger piloted by the delay circuit and supplying a pulse the duration of which is substantially less than the time of transit of a droplet between the deflecting electrodes.
 8. A device according to claim 7, comprising a delay store receiving orders for abrupt variations of the deflecting voltages, the store being ''''freed'''' by pulses from the monostable trigger to enable the application of the abrupt voltage variations to the electrodes during periods when no droplet is in transit between the deflecting electrodes. 